Precision measurements of the rotary position of a rotating element, such as a shaft in a limited rotation motor, are often required as, for example, when using a rotating element to control the angular position of a laser light source. Several different approaches have been proposed to do so, although most techniques have been found for one reason or another to be unsatisfactory when attempting to provide relatively high precision operation.
For example, techniques using variable differential transformers have been found deficient because of their excessive sensitivity to the presence of magnetic fields. Variable potentiometers have been found to have excessive mechanical hysteresis characteristics and, particularly when used around the balanced, or zero, region of the angular position, are subject to oscillation or jitter, which has resulted in contact wear problems. Moreover, potentiometers tend to be subject to general problems of limited life due to their mechanical motions.
Position detectors based on the use of light detection techniques suffer from problems of temperature instability, e.g., drifts, with time. Moreover, such light detection devices can be vibration sensitive and often produce substantially nonlinear operations, even over relatively small ranges of rotary motion of interest. Other optical devices using optical encoder techniques tend to have high mechanical inertia, poor resolution and are relatively expensive to manufacture.
Because of such problems, the art has generally turned to the use of capacitive sensing techniques. Such capacitive devivces can be fabricated to provide relatively high resolution with low noise characteristics and are also relatively insensitive to magnetic fields. Furthermore, capacitive sensors have relatively low rotary inertia, have no mechanical hysteresis, suffer substantially no wear problems, and can be manufactured at relatively low cost.
In currently used capacitive sensing devices, changes in capacitance are sensed as a result of the relative motion of metal plates forming the capacitance element, such capacitors normally using air as the dielectric medium. Such devices normally have one plate electrically grounded, which has sometimes made their use more difficult in many motor shaft applications. Moreover, such devices tend to be relatively sensitive to undesired radial motions of the rotating shaft itself, so that their accuracy is adversely affected in high precision measuring applications. Further, current capacitor sensor devices have less than desirable temperature stability, tending to drift with temperature changes. While the linearity characteristics thereof, which generally provide nonlinearities as low as 0.10%, for example, may be satisfactory for many applications, it is desirable that such linearity be improved by a least an order of magnitude for many high precision measurement applications.